Medial olfactory tract

Fig. 4 Odor map in the zebrafish olfactory bulb. Amino acids and nucleotides are received mostly by V2R-type olfactory receptors on microvillous OSNs and are represented in the lateral region of the OB. In contrast, bile acids are received by OR-type olfactory receptors on ciliated OSNs and are represented in the medial region of the OB. LOT lateral olfactory tract, MOT medial olfactory tract...

Different from mammals, in which the mitral cells send axons from the OB to the olfactory cortex through a single tract (the lateral olfactory tract, LOT), there are two major bundles in the secondary olfactory pathways of fish the LOT and the medial olfactory tract (MOT). From the results of neuroanatomical, electrophysiological, and behavioral experiments in cod, goldfish, and carp (Doving and Selset 1980 Stacey and Kyle 1983 Kyle et al. 1987 Sorensen et al. 1991 Hamdani et al. 2000, 2001a, b, 2002 Hamdani and Doving 2003, 2006 Weltzien et al. 2003), the... 

Hamdani EH, Stabell OB, Alexander G, Doving KB (2000) Alarm reaction in the crucian carp is mediated by the medial bundle of the medial olfactory tract. Chem Senses 25 103-109 Hamdani EH, Alexander G, Doving KB (2001a) Projection of sensory neurons with microvilli to the lateral olfactory tract indicates their participation in feeding behaviour in crucian carp. Chem Senses 26 1139-1144... 

Kyle AL, Sorensen PW, Stacey NE, Dulka JG (1987) Medial olfactory tract pathways controlling sexual reflexes and behavior in teleosts. Ann N Y Acad Sci 519 97-107 Laberge F, Hara TJ (2001) Neurobiology of fish olfaction a review. Brain Res Rev 36 41-59 Laberge F, Hara TJ (2003) Non-oscillatory discharges of an F-prostaglandin responsive neuron population in the olfactory bulb-telencephalin transition area in lake whitefish. Neuroscience 116 1089-1095... 

Sorensen PW, Hara TJ, Stacey NE (1991) Sex pheromones selectively stimulate the medial olfactory tracts of male goldfish. Brain Res 558 343-347 Sorensen PW, Fine JM, Dvornikovs V, Jeffrey CS, Shao F, Wang J, Vrieze LA, Anderson KR, Hoye TR (2005) Mixture of new sulfated steroids functions as a migratory pheromone in the sea lamprey. Nat Chem Biol 1 324-328... 

Wang JW, Wong AM, Flores J, Vosshall LB, Axel R (2003) Two-photon calcium imaging reveals an odor-evoked map of activity in the fly brain. Cell 112 271-282 Weltzien FA, Hoglund E, Hamdani EH, Doving KB (2003) Does the lateral bundle of the medial olfactory tract mediate reproductive behavior in male crucian carp Chem Senses... 

The neural pathways mediating alarm responses were examined in the crucian carp (Carassius carassius L.). In these fish, two olfactory tracts convey information from the olfactory bulbs [adjacent to the olfactory organs (i.e., nostrils)] to other parts of the brain. One courses along the midline (the medial olfactory tract) and the other along the side (the lateral olfactory tract). The medial olfactory tract further divides into two bundles (the medial and the lateral bundles of the medial olfactory tract). Severing the medial bundle of the medial olfactory tract eliminated the alarm responses to skin extract, whereas severing the lateral bundle of the medial olfactory tract diminished the feeding behavior (Hamdani et al. 2000). 

Upon entering the forebrain, the medial olfactory tract subdivides and innervates terminal fields in the contralateral olfactory bulb, telen ... 

Based on the observation that section of the medial olfactory tract also altered bulbar electrical activity, Kara and Gorbman (1967) suggested that some of the effects of sex steroids on the bulb could be mediated by efferent fibres from the forebrain. Subsequently, it was found that the ventral telencephalon (medial terminal field Vs-pVv area) of the goldfish has both afferent and efferent connections with the olfactory bulbs (Oka et al.,... 

Fig. 3 Vomeronasal system. Schematic representation of a rodent nasal cavity and brain (lateral view). Accessory olfactory bulb (AOB) mitral cells project to vomeronasal and extended amygdala. Inset The VNO is a bilateral tubular structure located at the base of the nasal septum. VSNs that express the same V1R or V2R converge on a small number of glomeruli in the AOB. Sensory neurons located in the apical layer of the epithelium project to the anterior part of the AOB, whereas those present in the basal layer project to the posterior part. MOE main olfactory epithelium, MOB main olfactory bulb, BSTMPM posteromedial bed nucleus of the stria terminalis, MEA medial amygdaloid nucleus, BACfF bed nucleus of the accessory olfactory tract, PMCO posteromedial cortical amygdaloid area...

Abbreviations ACo = anterior cortical amygdaloid nucleus AOB = accessory olfactory bulb Me = medial amygdaloid nucleus AON = anterior olfactory nucleus (m = medial division) PCo = posterior cortical amygdaloid nucleus BST = bed nucleus of the stria terminalis DHR = dorsal hippocampal rudiment DPC = dorsal peduncular cortex DR = dorsal raphe nucleus Ent = entorhinal cortex LC = locus coeruleus LPO = lateral preoptic area MOB = main olfactory bulb MnR = median raphe BAOT = nucleus of the accessory olfactory tract NLOT = nucleus of the lateral olfactory tract DB = nucleus of the diagonal band PeCo = periamygdaloid cortex Pir = piriform cortex Tu = olfactory tubercle TT = taenia tecta... 

ISH-taenia tecta, AON (few), olf. tubercle, piriform (layer II, some III) cells associated with medial olf. tract, olf. tubercle all olfactory regions... 

The AOB has direct projections to the amygdala, specifically to the medial and posterior cortical nuclei, the bed nucleus of the stria terminalis and the nucleus of the accessory olfactory tract. These pathways may be involved in the processing of pheromonal information. Neurons in the AOB targets express gonadal steroid receptors and thus may be modulated directly by circulating hormones. The efferent connections of the accessory olfactory system are summarized in Fig. 22. 

There are major differences between centrifugal inputs to MOB and AOB. First, centrifugal inputs to AOB arise from far fewer brain regions than inputs to MOB. The major afferents to AOB are from the bed nucleus of the stria terminalis, the nucleus of the accessory olfactory tract, the medial amygdala nucleus and the posteromedial cortical amygdala nucleus (De Olmos et al. 1978 Shipley and Adamek, 1984). A restricted part of the medial division of AON sends a dense projection to the granule cell layer of AOB (Rizvi et al. 1992), but all other divisions of AON lack connections with AOB. 

MeAa medial nucleus of the amygdala, anterior part MeApd medial nucleus of the amygdala, posterodorsal part MeApv medial nucleus of the amygdala, posteroventral part NLOT nucleus of the lateral olfactory tract ONL olfactory nerve layer... 

Figure 2. Schematic diagram of the nasal cavities and forebrain of a salamander, illustrating the central projections of the olfactory and vomeronasal systems in dorsal view. Anterior is toward the top of the figure, and only ipsilateral projections are shown. The medial (A) and lateral (B) olfactory tracts arise from the olfactory bulb. (C) The extra-bulbar ol ctory pathway bypasses the olfactory bulb and projects directly to the anterior preoptic area. (D) The accessory olfactory bulb, which receives input from the vomeronasal organ, projects to the lateral amygdala (la). Other abbreviations apoa = anterior preoptic area dp = dorsal pallium Ip = lateral pallium mp = medial pallium ma = medial amygdala s = septum sir = striatum. Based on descriptions in Hetrick, 1927,1933,1948 Kokoros and Northcutt, 1977 and Schmidt and Roth, 1990.

Since spawning behavior and production and release of milt in male goldfish are influenced by the medial but not by the lateral olfactory tract, any terminal fields that are innervated by the medial tract, but not by the lateral tract, are likely candidates for mediating the effects of pheromones. Two such areas occur in the goldfish the medial terminal field of the ventromedial telencephalon and the anterior preoptic area (Ichikawa and Ueda, 1978 von Bartheld et al., 1984), both of which are implicated in the control of reproductive events. 

Central/Tertiary structures The fish olfactory bulb is a fourlayered structure much as in higher vertebrates. Within the 2nd layer, the first synapse for olfactory input is on the dendrites of the mitral cells (MC). About 1000 ORN axons converge on one MC, a ratio similar to mammals. The MC output, from cells at various levels, leads into several glomeruli and receives (inhibitory) input from granule cells. The latter also innervate a distinct cell type in the MC layer of teleosts — the ruffed cells (RC), with which they have reciprocal synapses [Fig. 2.18(a)] both relay cells send ascending fibres to forebrain centres (Kosaka and Hama, 1982). The RC are unlike the MC since they are not stimulated by the ORNs directly. Their interactions (Chap. 5) may contribute to the processing of pheromonal stimuli (Zippel, 2000). The main bulbar pathways project to several nuclei in the forebrain via two ipsilateral tracts, the lateral and medial [Fig. 2.18(b)], the latter mediates sexual behaviour and the former probably other behaviours (Hara,... 

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